It has been recently proposed that a particulate filter for capturing particulates in exhaust gas is incorporated in an exhaust pipe and a selective reduction catalyst capable of selectively reacting NOx with ammonia even in the presence of oxygen is arranged downstream of the particulate filter, urea water as reducing agent being added between the selective reduction catalyst and the particulate filter, thereby attaining lessening of both the particulates and NOx.
Such addition of the urea water to the selective reduction catalyst is conducted between the particulate filter and the selective reduction catalyst. Thus, in order to ensure sufficient reaction time for pyrolysis of the urea water added to the exhaust gas into ammonia and carbon dioxide gas, it is necessary to prolong a distance between a location where the urea water is added and the selective reduction catalyst. However, such arrangement of the particulate filter and the selective reduction catalyst substantially spaced apart from each other will extremely impair the mountability on a vehicle.
In order to overcome this, a compact exhaust emission control device as shown in FIG. 1 has been proposed. In the exhaust emission control device shown, incorporated in an exhaust pipe 4 through which exhaust gas 3 flows from a diesel engine 1 via an exhaust manifold 2 is a particulate filter 5 housed in a casing 7 to capture particles in the exhaust gas 3; arranged downstream of and in parallel with the particulate filter 5 and housed in a casing 8, is a selective reduction catalyst 6 having a property capable of selectively reacting NOx with ammonia even in the presence of oxygen. A discharge end of the particulate filter 5 is connected to an entry end of the selective reduction catalyst 6 through an S-shaped communication passage 9 with an injector 11 at its upstream side such that the exhaust gas 3 discharged through the discharge end of the particulate filter 5 is turned down into the entry end of the adjacent selective reduction catalyst 6.
The mixing pipe 10, which provides a straight portion of the communication passage 9, has the entry end coaxially provided with the injector 11 so as to add the urea water centrally of the mixing pipe 10. For example, as shown in FIG. 2, a side surface of the entry end in the mixing pipe 10 adjacent to the discharge side of the particulate filter 5 is formed with an opening 12 into which the exhaust gas 3 is tangentially introduced by, for example, guide fins 13 so that a resultant swirling flow formed in the mixing pipe 10 can facilitate dispersion of the urea water added by the injector 11.
In FIG. 1, an oxidation catalyst 14 for oxidization treatment of unburned fuel in the exhaust gas 3 is arranged in the casing 7 and in front of the particulate filter 5. With an accumulated amount of the particulates being increasing, fuel is added to the exhaust gas 3 upstream of the oxidation catalyst 14 so that the added fuel undergoes oxidation reaction during its passage through the catalyst 14, the exhaust gas 3 being heated by a resultant reaction heat to increase a catalyst floor temperature of the particulate filter 5 just behind, whereby the captured particulates may be actively burned out to attain forced regeneration of the particulate filter 5.
Arranged in the casing 8 and behind the selective reduction catalyst 6 is an ammonia lessening catalyst 15 for oxidization treatment of surplus ammonia having passed through the reduction catalyst 6 without reaction. Thus, ammonia is prevented from being discharged to outside of the vehicle while entrained in the exhaust gas 3.
With such construction being employed, particulates in the exhaust gas 3 are captured by the particulate filter 5 and the urea water is added by the injector 11 into the exhaust gas 3 at the entry end of the mixing pipe 10 and decomposed into ammonia and carbon dioxide gas, so that NOx in the exhaust gas 3 is favorably reduced and depurated on the selective reduction catalyst 6 by the ammonia to thereby attain lessening of both the particulates and NOx in the exhaust gas 3.
In this case, the exhaust gas 3 discharged through the discharge end of the particulate filter 5 is turned down by the communication passage 9 into the entry end of the adjacent selective reduction catalyst 6. As a result, enough reaction time is ensured for production of ammonia from the urea water since a long distance is ensured between the location where the urea water is added by the injector 11, and the selective reduction catalyst 6.
Moreover, the particulate filter 5 and the selective reduction catalyst 6 are arranged in parallel with each other and the communication passage 9 is arranged between and along the filter 5 and the catalyst 6, so that the whole structure becomes compact in size to substantially enhance its mountability on a vehicle.
As to a prior art literature pertinent to the invention, there already exists, for example, the following Patent Literature 1.
[Patent Literature 1] JP 2005-155404A